The morphological complexity of mammals, as compared to invertebrates, is thought to have arisen through advantageous genetic changes that occurred during the course of evolution. A new research study published in the September issue of Developmental Cell suggests that the evolution of higher-order vertebrate organ systems can result from primitive developmental genetic programs that are, in a sense, recycled for entirely new structures.
According to the expression patterns in the fruit fly, Drosophila, the ancestral action of the Hmx gene was limited to the development of the central nervous system (CNS). Dr. Thomas Lufkin from the Genome Institute of Singapore and colleagues show that the mouse Hmx2 and Hmx3 genes have apparently interchangeable functions in CNS development and have overlapping yet distinct functions in the development of the vestibular system of the inner ear, an organ that has no counterpart in Drosophila. The researchers found that when mice are genetically engineered to lack Hmx2 and Hmx3, Drosophila Hmx can substitute for the mouse Hmx3 gene in CNS development and, surprisingly, can also direct development of the inner ear. Therefore, a Drosophila gene can direct formation of an organ system that does not even exist in the Drosophila body.
“These results demonstrate that the evolution of higher vertebrate characteristics can result from the recycling or redeployment of ‘old’ genes in new parts of the embryo, rather than through mutation of gene protein-coding sequence alone. Old genes can be given a new purpose through ‘reassignment’ to organs undergoing evolutionary advancement. The reassignment likely comes through a shuffling of existing regulatory elements to generate new combinations that are specific to the new organ,” explains Dr. Lufkin.
Weidong Wang, J. Fredrik Grimmer, Thomas R. Van De Water, and Thomas Lufkin: “Hmx2 and Hmx3 Homeobox Genes Direct Development of the Murine Inner Ear and Hypothalamus and Can Be Functionally Replaced by Drosophila Hmx”
Developmental Cell, Volume 7, Number 3, September 2004, pages 439-453.